The invention relates generally to x-ray tubes and, more particularly, to a method of fabricating a high-voltage insulator for x-ray tubes. The invention is described with respect to an x-ray system, but one skilled in the art will recognize that the invention may be used in, for instance, electron tubes or other devices in which high voltage instability occurs.
X-ray systems typically include an x-ray tube, a detector, and a gantry to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes may include a rotating anode structure for the purpose of distributing heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, the anode assembly is typically rotated at high rotational speed.
Newer generation x-ray tubes have increasing demands for providing higher peak power and higher accelerating voltages. For instance, x-ray tubes used in medical applications typically operate at 140 kV or more, while 200 kV or more is common for x-ray tubes used in security applications. However, one skilled in the art will recognize that the invention is not limited to these voltages, and applications requiring greater than 200 kV may be equally applicable. At these voltages, x-ray tubes are susceptible to high-voltage instability and insulator surface flashover which can reduce the life expectancy of the x-ray tube or interfere with the operation of the imaging system.
In a typical x-ray tube, there is a disk-shaped ceramic insulator having an opening for electrical feeds therein. The cathode post, or conduit for the electrical feeds, typically houses three or more electrical leads for feeding voltage to the cathode. Typically, the insulator, at its center opening, is attached to the cathode post which may structurally support the cathode. The cathode typically includes one or more tungsten filaments. At its perimeter, the insulator is typically hermetically connected to a cylindrical frame, which houses a vacuum chamber in which the anode and the cathode are typically positioned.
X-ray tubes may operate at up to 100 kW peak power, and at an average power of 5 kW for hours at a time. X-ray tubes are susceptible to high-voltage stresses at the junctions between the insulator and center cathode support structure, and between the insulator and x-ray tube frame. These junctions are commonly referred to as triple-point junctions describing the intersection of metal, dielectric, and vacuum. Triple-point junctions are common sources of high-voltage instability due to field emission of electrons that can reduce the life expectancy of the x-ray tube.
Imperfections on the insulator surface in the vacuum region can include particles of surface contamination, pores or voids, and grooves and pits from machining and may lead to secondary electron emission. This occurs when field emitted electrons strike the insulator surface, releasing more electrons into the vacuum region. A cascading effect can lead to electrical arcing and insulator surface flashover. The potential for insulator surface flashover in an x-ray tube may be reduced by decreasing the intensity of the electric field at the insulator surface near the triple-point junction and by eliminating the imperfections along the insulator surface that contribute to secondary electron emission.
Blasting an insulator surface with steel or glass beads can clean the surface and reduce surface roughness to roughly 1-3 microns. This method may reduce secondary electron emission and the likelihood of insulator surface flashover, enough for most low-voltage x-ray tube applications. For high-voltage applications, mechanical polishing or electropolishing offers better results than surface blasting by reducing surface roughness to 0.05 to 0.2 microns. But even using these improved production methods, the insulators are still susceptible to electrical breakdown at higher operating voltages.
Computed tomography (CT) systems represent an advanced application of x-ray tube technology. To improve the functionality of CT imaging, greater demands are placed on x-ray tubes. The need to increase patient throughput puts a premium on reducing scan times. The combination of shorter scan times and higher patient loads often translates into higher operating voltages and more frequent use for CT system x-ray tubes further increasing the potential for electrical breakdown.
Therefore, it would be desirable to have a method of fabricating a high-voltage insulator for an x-ray tube or vacuum tube that is resistant to insulator surface flashover caused by field emission and secondary electron emission.